1. Technical Field
The present invention relates to a controlling circuit, and particularly to a controlling circuit for outputting a square wave signal to control a fan rotating speed.
2. Related Art
Recent years, with wide prevalence and vigorous development of semiconductor technology, various electronic devices have boomed on market. Since some waste heat may generate in operation, a fan is typically provided to exhaust the heat.
Generally, a conventional fan is controlled by controlling a drive signal so that the fan exhausts the heat at a full speed. However, the waste heat generated from the electronic device varies with different loads. If the fan operates at the full speed, not only lifetime of the fan reduces, unnecessary waste of electricity source may be unavoidable, the issue of incapability of fan rotating speed control arises correspondingly.
In response, there has been a manner of transmitting a pulse width modulation by using a control chip proposed. However, in the case where the control chip for transmitting the pulse width modulation is pulled out or damaged, the fan may stop operating or cannot continue to adjust a duty-ratio of a square wave signal as desired to control the fan rotating speed.
In view of the above, it may be known that there has been the issue in the prior art that the duty-ratio of the square wave signal may not be adjusted as desired. Therefore, there is quite a need to set forth an improvement means to settle down this problem.